X-ray crystallographic techniques are being used to study some acridine, benz(a)anthracene and chrysene derivatives that are frameshift mutagens, carcinogens and/or antitumor agents. Studies will be made of metabolic products of carcinogenic polycyclic aromatic hydrocarbons, such as arene oxides and dihydrodiols. Molecular complexes of the hydrocarbons with more polar planar molecules will also be studied. Trinitrobenzene, pyromellitic dianhydride and naphthalene 1,4,5,8-tetracarboxylic acid dianhydride are presently being used as complexing agents. Studies of the mechanisms of some enzymes, particularly those of the Krebs cycle, are being made by detailed studies of substrates, inhibitors and the enzymes themselves. Work on substrates and inhibitors is concentrated on enzymes utilizing citrates (citrate synthase, citrate lyase, ATP citrate lyase and aconitase) and on delta5-3-ketosteroid isomerase. Studies on crystalline enzymes involve work on xylose isomerase, citrate synthase and aconitase.